Your question goes in a couple of different directions, but lets see:
Option #1 - Soldering galvanized steel
Soldering galvanized steel is very simple, yet, if compared to copper is a little trickier to get done. First, lets talk about the flux you should use.
My favorite method is a mixture 50/50 of 2 fluxes:
Muriatic acid: link at Amazon
Soldering fluid: link at Amazon
I actually prefer Johnson's (here), but I don't think you can buy it directly.
1 - Mix 50% / 50% of both acids.
2 - Prepare the surface of the galvanized steel. This is not "necessary", but makes your life easier. Scrub the surface with a wire brush, or sand it down, scratch it, anything to make it less smooth to help the solder grip. Cheap Example:
Wire brush: Link at Amazon
3 - Have your soldering iron hot enough to melt solder (50/50 solder is fine).
4 - Solder like you would copper, except, make sure the flame is either turned of on your soldering iron, or that it doesn't make contact with the galvanized steel for too long. Basically, you have to be quick. If the flame burns the galvanized coating, it will turn black, and it'll make soldering extremely hard from that point on.
5 - You may want to grab an acid cup and brushes.
Option #2 - Welding galvanized steel
Welding galvanized steel is very possible. It is not as easy as welding plain steel, however, it is not that hard. One thing to note is that the smoke from welding galvanized steel is very toxic, and you should be prepared to take precautions to not get sick. You can then spray paint the area welded with some zinc pray for rust protection.
Option #3 - Boxing a panel
For metal roofing, specially standing seam (which I don't think its what you're referring to), many people simply don't do this step, and it is honestly not even recommended by some metal roof manufacturers, which is mind boggling to me. Some recommend to simply seal a joint or do other things with a z flashing for example and sealer. I'm only mentioning this because, the solution proposed by "bib" is the very best case scenario, since there is no cutting and soldering to waterproof. This should be always the go to method, whenever there is an opportunity to do so.